FIG. 12 is a cross-section showing an example of a conventional automotive alternator. The alternator in FIG. 12 includes: a case 3 consisting of an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 disposed in the case 3 so as to rotate freely by means of bearings to one end of which a pulley 4 is secured; a Lundell-type rotor 7 secured to the shaft 6 and housed in the case 3; fans 5 secured to both ends of the rotor 7; a stator 8 secured to the inner wall of the case 3 so as to surround the rotor 7; slip rings 9 secured to the other end of the shaft 6 for supplying electric current to the rotor 7; a pair of brushes 10 disposed in the case 3 so as to slide in contact with the slip rings 9; a brush holder 11 accommodating the brushes 10; a rectifier 12 electrically connected to the stator 8 for rectifying alternating current generated in the stator 8 into direct current; a heat sink 17 fitted over the brush holder 11; and a regulator 18 attached to the heat sink by adhesive for regulating the magnitude of the alternating current generated in the stator 8.
The regulator 18 is constructed by mounting onto a ceramic board power transistors for controlling the excitation current flowing to the rotor 7 and other control circuits. Then, the heat sink 17, which has a plurality of fins, is fixed to the reverse side of the ceramic board (the side on which the power transistors and control circuits are not mounted) using adhesive, so as to radiate heat generated by the power transistors.
As shown in FIGS. 13 and 14, the rectifier 12 includes a positive-side heat sink 24 to which a plurality of positive-side diodes 23 functioning as unidirectional conducting elements are joined, and a negative-side heat sink 26 to which a plurality of negative-side diodes 25 functioning as unidirectional conducting elements are joined, and a circuit board 27. The positive-side and negative-side heat sinks 24, 26 each has a plurality of straight fins 24a, 26a projecting perpendicular to the shaft 6 and extending parallel to the shaft 6. For example, twenty of these fins 24a , 26a may be provided having an average thickness of 1.3 mm in the direction of projection, a pitch of 2.5 mm, and a projecting height of 14 mm. The plurality of diodes 23, 25 are joined with predetermined spacing by soldering to the surfaces of the heat sinks 24, 26, respectively, parallel to the shaft 6 on the opposite side to the side on which the fins 24a, 26a are disposed. The heat sinks 24, 26 are assembled so that the backs of each of the diodes 23, 25 are positioned opposite each other in the radial direction. Leads 23a, 25a of the paired positive-side and negative-side diodes 23, 25 are gathered together in one place at the connecting terminals 27a of the circuit board 27, and are each connected to the output terminals 16a of the stator coil 16, so as to rectify three-phase alternating current into direct current. Furthermore, heat generated by the diodes 23, 25 due to power generation is radiated from the fins 24a, 26a disposed on the heat sinks 24, 26.
The rotor 7 includes: a rotor coil 13 for generating magnetic flux by passing electric current therethrough; and a pole core 14 disposed so as to cover the rotor coil 13 in which magnetic poles are formed by the magnetic flux generated by the rotor coil 13. The pole core 14 includes a first pole core body 21 and a second pole core body 22 which mutually interlock.
The stator 8 includes: a stator core 15; and a stator coil 16 composed of wire wound onto the stator core 15 in which an alternating current is generated by changes in the magnetic flux from the rotor coil 13 as the rotor 7 rotates.
In a conventional automotive alternator constructed in the above manner, current is supplied by a battery (not shown) through the brushes 10 and slip rings 9 to the rotor coil 13, whereby magnetic flux is generated, and at the same time, the rotational torque of the engine is transferred to the shaft by means of the pulley 4, rotating the rotor 7 so that a rotating magnetic field is imparted to the stator coil 16 and electromotive force is generated in the stator coil 16. This alternating electromotive force passes through the rectifier 12 and is rectified into direct current, the magnitude thereof is regurated by the regulator 18, and the battery is recharged.
Now, the rotor coil 12, the stator coil 16, the rectifier 12, and the regulator 18 constantly generate heat during power generation. In an alternator with a rated output current in the 100 A class, the amount of heat generated in the rotor coil 12, the stator coil 16, the rectifier 12, the regulator 18 is 60 W, 500 W, 120 W, and 6 W, respectively.
Intake openings 1a, 2a and exhaust openings 1b, 2b for allowing ventilation generated by the fans 5 disposed on the rotor 7 to pass through are bored in the front bracket 1 and the rear bracket 2. Thus, in the rear end, due to the rotation of the fans 5 (rotor 7), air from outside flows into the case 3 through the intake openings 2a disposed opposite the heat sinks 17, 24, 26, flows through the heat sinks 17, 24, 26 and cools the rectifier 12 and the regulator 18. Then that air is redirected centrifugally by the fans 5, cools the stator coil ends in the rear end and is then discharged to the outside through the exhaust openings 2b. In the front end, due to the rotation of the fans 5, air from outside flows axially into the case 3 through the intake openings 1a, then that air is redirected centrifugally by the fans 5, cools the stator coil ends in the front end, and is then discharged to the outside through the exhaust openings 1b.
The fins disposed on the heat sinks 17, 24, 26 are formed perpendicular to the contact surfaces between the ceramic board and diodes and the heat sinks 17, 24, 26, and temperature increases in the diodes 23, 25 of the rectifier 12 and the power transistors of the regulator 18 are suppressed by heat exchange with the air flowing between the fins. When the amount of heat generated and the materials used are constant, the value of the temperature increase dt is greatly dependent on the speed v of the air flowing through the heat sinks and the surface area A of the fins, and the relationship between them can be expressed by Expression (1): EQU dt.varies.Q/(A.times.v.sup..alpha.) (1)
Moreover, .alpha. is determined by the state of the air flowing through the heat sinks, and is 0.5 if the flow is laminar and 0.8 if the flow is turbulent.
From Expression 1, it can be seen that temperature increases dT can be suppressed by increasing the surface area A of the fins, and in a limited space, this means making the fins thinner and increasing the number of fins. However, although the surface area of the fins can be increased by making the fins thinner and increasing the number of fins, the amount of ventilation passing through decreases, reducing the speed v of the air, and consequently temperature increases cannot be suppressed in this way.
FIG. 15 is a cross-section showing another example of a conventional automotive alternator such as that described in Japanese Patent Laid-Open No. HEI 8-182279, FIG. 16 is a planar projection of the rear bracket of the automotive alternator shown in FIG. 15, and FIGS. 17 and 18 are a perspective and a plan, respectively, showing a rectifier used in the automotive alternator shown in FIG. 15.
Apart from the use of a rectifier 30, the construction of this conventional automotive alternator is the same as for the conventional automotive alternator shown in FIG. 12.
The rectifier 30 used in this conventional automotive alternator includes a positive-side heat sink 31 to which a plurality of positive-side diodes 23 are joined, and a negative-side heat sink 32 to which a plurality of negative-side diodes 25 are joined, and a circuit board 33. The heat sinks 31, 32 and the circuit board 33 are each formed in a horseshoe shape. A plurality of fins 31a are disposed radially on one surface of the positive-side heat sink 31. The plurality of diodes 23 are joined with predetermined spacing by soldering to the surface of the heat sink 31 on the opposite side to the side on which the fins 31a are disposed. On the other hand, no fins are disposed on the negative-side heat sink 32 and the plurality of diodes 25 are joined with predetermined spacing by soldering to the main surface thereof. The heat sinks 31, 32 are coaxially assembled so that the surfaces mounted with the diodes 23, 25 lie on the same plane, and the leads 23a, 25a of each of the paired diodes 23, 25 are positioned so as to be opposite each other in the radial direction. Then, the leads 23a, 25a of the paired diodes 23, 25 are gathered together in one place at the connecting terminals 33a of the circuit board 33, and are each connected to the output terminals 16a of the stator coil 16.
In a rectifier 30 constructed in this manner, the surfaces of the heat sinks 31, 32 mounted with the diodes 23, 25 are perpendicular to the axis of the shaft 6, and are mounted to the rear bracket 2 so as to be coaxial to the shaft 6. Here, the negative-side heat sink 32 is directly mounted to a bearing surface in the rear bracket 2, and is grounded. Furthermore, the fins 31a project parallel to the axis of the shaft 6 at a right angle from one surface of the heat sink 31, and form a radial pattern extending towards the axis of the shaft 6.
In this conventional automotive alternator, because the surface of the heat sink 31 mounted with the diodes is perpendicular to the axis of the shaft 6, air introduced through the intake openings 2a opposite the fins 31a by the rotation of the fans 5, flows between the fins 31a, then passes between the heat sink 31 and the shaft 6 and enters the fans 5. Because this heat sink 31 is formed in a horseshoe shape, the spaces between the fins 31a are narrower on the side closest the shaft 6. Thus, if the fins 31a are made thinner and the number thereof is increased, the flow of air is choked on the side closest to the shaft 6, leading to loss of cooling performance.
These heat sinks 24, 26, 31, 32 are usually prepared by a die cast manufacturing process, and if the fins are made too thin, misruns and die removal problems occur, making it impossible to prepare the heat sinks. Conversely, if the thickness of the mold corresponding to the spacing between the fins is too thin relative to the thickness of the fins, the working life of the mold is significantly reduced compared to a normal die with which it is possible to mold 200,000 shots.